Nucleophilic Substitution Enables MXene Maximum Capacitance and Improved Stability.

Combining the merits of battery and supercapacitor into a single device represents a major scientific and technological challenge. From a design perspective, electrode material plays a key role in the device and the fundamental difficulty lies in incorporating a high density of active sites into a stable material with excellent charge transfer kinetics. Here, the synthesis is reported of a nearly full‐oxygen‐functionalized 2D conductive transition metal carbide (Ti3C2Oy) with ultrahigh density of Ti─O/═O redox‐active sites by nucleophilic substitution and in situ oxidation under the presence of a proper electrophilic reagent (K+). The fabricated electrode delivered exceptionally high gravimetric and volumetric capacitance (1,082 F g−1 and 3,182 F cm−3 in a potential window of 0.85 V, approximating the theoretical capacity of many transition metal oxides), fast charging/discharging in tens of seconds across a wide range of temperature (−70 to 60 °C), and excellent structural and chemical stability. These promising results provide avenues for the development of high‐energy, high‐power storage devices as well as electromagnetic shielding, and electronic and optoelectronic devices. [ABSTRACT FROM AUTHOR]